You can't tel which eye he used since both were open. Itachi put out amaterasu despite not having kagutsutchi this clearly means putting out amaterasu has nothing to do with kagutsuchi
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Personal reasoning? My argument has merit, you just read the last statement without considering the rest of the content. Naruto pulled out jutsu from thing air prior to RSM, unless you are claiming that he had universal comprehension since the war started.
Naruto couldn't use their power because the bijuu couldn't manifest inside his consciousness before he gained RSM unless he was in physical contact with them. If naruto using the bijuu chakra in RSM was due to universal comprehension then he wouldn't need their help to use the special techniques.
That's because most of his RSM powers discounting the TSB and elemental techniques are the same as his BM powers. The chakra cloak, chakra arms, avatar etc. He didn't need that much training to use them. TSB can easily be mastered because naruto already knew how to shape chakra through rasengan and FRS training. As for nature transformation, naruto prior to RSM was able to alter his chakra to match that of anyone else, clearly nature transformation wouldn't be a problem for him. The only things I believe RSM granted him besides extra power and sensing are TSB, flight and an affinity for 5 basic elements and yin-yang release. It didn't grant him any form of supernatural cognizance or the like.
Not everything in the DB is wrong, but a lot of parts are.